The clasic internal combustion engine operates with constructive uniform piston strokes, thus determining a single value for the torque at maximum output, this being calculated so as to surpass the highest stress to be encountered by the engine, representing a disadvantage because the load torque variation encountered by the engine of a vehicle in operation, does not correspond in an economically proportional variation of the fuel consumption.
Another disadvantage of the above mentioned engine is represented by the existence of the crankshaft as a machine element in the mechanism that converts linear motion into rotary motion, creating for the piston some radial component forces in the rotation plane of the crankshaft, these radial forces causing the pistons and cylinders to wear and also decrease the mechanical efficiency.
At the same time, another shortcoming of the classic internal combustion engine is represented by the relatively long linkage between the cam shaft and the intake and exhaust valves.
The lack of an elastic element within the linkage of the mechanism converting linear motion into rotary motion constitutes another disadvantage of the classic engine; that elastic element would damp the shocks produced by the explosions of the fuel mixture within the combustion chamber, thus provides a an improvement in the endurance limit of the machine elements as well as, esspecially with Diesel and other fast engines, diminish the rocking along the piston working axis.
Another internal combustion engine is known, having variable piston displacement, in which the variation of the piston displacement is obtained both by changing the length of the piston stroke - using a change in the actual length of the crank driven by the piston - and by correspondingly changing the distance between the cylinder head and the crankshaft, getting the desired compression ratio.
The variation of the actual length of each crank is obtained by means of an eccentric bushing interposed between the crankshaft and the connecting rod, with the large end thereof encasing it, and by means of a mechanism adjusting the bushing position as to the crank.
Each cylinder head of that engine can move axially inside its cylinder and is coupled to a control mechanism contiguous with a mechanism which modifies the stroke length - so that the distance between the cylinder head and the crankshaft varies with the stroke, the adjustment of the mechanism being mannually or automatically done.
As far as the valves are concerned, the rocker arms are mounted on the mobile head of each cylinder in order to monitor the opening of the valves, they being mounted swivelably on eccentrics interlocked with a shaft mounted on a fixed body and connected to a mechanism which moves the cylinder head nearer to or further from the crankshaft so that the virtual oscillating axis of the rocker arms can be displaced according to the cylinder head.
The above mentioned engine having variable piston displacement has the disadvantage of a very complicated construction, increasing the possibility for insignificant wear in the linkage adjusting the stroke, causing major misadjustments of the engine operation. The mobile cylinder head presents special machining and operation problems (that is, sealing and cooling problems). At the same time, the mechanical efficiency of that engine is very low.